The present invention relates to an exhaust gas recirculation system for controlling the flow of exhaust gas from an exhaust gas passage to an engine intake passage of an internal combustion engine, and more particularly, to an actuator assembly for an exhaust gas recirculation system.
Although the use of the present invention is not limited to any particular type of engine, its use is especially advantageous in connection with a diesel engine, for reasons which will become apparent subsequently.
Typically, exhaust gas recirculation (EGR) valves have been disposed between the engine exhaust manifold and the engine intake manifold, and have been operable, when in the open position, to permit the recirculation of exhaust gas from the exhaust side of the engine back to the intake side. As is well known to those skilled in the art, such recirculation of exhaust gasses is helpful in reducing various engine emissions.
An EGR system including an electrically operated type actuator is illustrated and described in U.S. Pat. No. 5,606,957. The actuator for the valve stem in the cited patent is a stepper motor, which is generally satisfactory in performing the basic function of opening and closing the EGR valve. However, in most vehicle applications for EGR valves, and especially in diesel engine applications, it must be possible to close the EGR valve within about fifty milliseconds of the time the closing command is generated, and to open the EGR valve within about 100 millisecond of the time the opening command is generated, and the device of the cited patent is not typically capable of such rapid opening and closing.
Co-pending application U.S. Ser. No. 08/881,622, Filed Jun. 24, 1997 in the names of Edwin D. Lorenz, Glen R. Lilley and David Turner for an "EGR SYSTEM AND IMPROVED ACTUATOR THEREFOR", assigned to the assignee of the present invention, teaches an improved actuator for an EGR valve. In the co-pending application, the actuator includes an electric motor of the relatively high-speed, continuously rotating type, such as a permanent magnet DC commutator motor, which provides a relatively high-speed, low torque output. The actuator also includes a reduction gear train, the output of which is a relatively high torque, low speed rotation of an output gear which is connected by a suitable linkage to the stem of the EGR valve. Although the actuator of the co-pending application provided generally satisfactory performance, and is able to open and close the EGR valve within the required time periods, the reduction gear train set forth therein resulted in excessive size, complexity and cost in the overall actuator assembly.
U.S. Pat. No. 4,690,119 shows an actuator for an EGR valve in which the output of a motor, such as a stepper motor, is transmitted by means of an output gear to an actuator assembly including a sector gear. Pivotal movement of the sector gear in response to operation of the motor moves the EGR valve through a linkage member. In the device of the cited patent, which appears to be intended for use with only an Otto cycle engine, the various flow passages and the valve seat are arranged such that the pressure of exhaust gas in the exhaust manifold would tend to bias the EGR valve toward an open position. Thus, the performance criteria for the actuator to open the EGR valve, assisted by the gas pressure, are not especially severe, and in the cited patent, the sector gear is arranged to provide a relatively constant level of torque and speed when opening the EGR valve.
Many diesel engines are turbo charged, which means that the exhaust gas pressure in the exhaust manifold must be substantially above atmospheric pressure, in view of the fact that it is the pressure in the exhaust manifold which drives the input (impeller) of the turbo charger. In such turbo charged diesel engines, it is considered necessary for the EGR valve to be of the "pressure biased closed" type rather than being of the "pressure biased open" type, as is the device shown in above-cited U.S. Pat. No. 4,690,119. If the EGR valve of a turbo charged diesel engine were of the "pressure biased open" type, the elevated exhaust manifold pressures would result in substantial leakage of exhaust gas into the intake manifold, at times when such leakage is not desirable. However, by making the EGR valve of the pressure biased closed type, there is a much greater "opening force" required for the actuator assembly.